Using an in vivo micro-fluorimetry system previously developed, we have employed carboxy fluorescein and fluorescently-tagged dextran tracers in an attempt to characterize the mass transport characteristics of solid tumors. Some of the tissue signals could be analyzed with a simple linear model by deconvolution with the associated blood signals to provide intercompartmental transport parameters; others showed a more complex behavior. Comparison of our transport data with histological studies of random tissue samples support the hypothesis that tumour mass transport is spatially heterogeneous.